The first thing you think about when seeing low-value Rs [eg, 10 ohms]is "lotsa power dissipation", meaning the Rs and also the NPN need tobe largish parts.

Full power into the R would mean 9V*9V/10ohms = 8.1 watt. In anything but a well-designed high power ckt, that's a lot of heat.

Also, in this case, the NPN can conceivably dissipate a lot of heat too. Worstcase is for 9V/2 = 4.5V on the collector, which would mean Ic = 4.5V/10ohms= .45A, and PD(npn) = 4.5V * .45A = 2 watts. A little TO-92 device can only dissipate 0.3 watts or so before overheating. Even a TO-220 case will get prettywarm with PD = 2 watts, and you need to think about heatsinking even here.

Then, the other thing that happens is, when the transistor gets hot, hFE changes a great deal too. hFE is so variable, all in all, that good ckts are designed to take this into account.

dlhylton

To test a transistor, you can do simple resistance tests on the various diode junctions, as illustrated here, http://www.learningaboutelectronics.com/Articles/How-to-test-a-transistor. Measure each of the pairs of diode junctions. Collector-emitter, collector-base, base-emitter. Read the resistance of one junction and then read the same junction with the polarity probes switched. One side should read very high resistance, over 1 megohms. And the other should read a moderate resistance, a few hundred thousand ohms. If this is the case for all three junctions, the transistor should be a good working one.